1. Field of the Invention
The present invention relates to a manufacturing method and a designing method for a field emission display which performs display by emitting electrons from a cathode substrate to a fluorescent screen glass which are disposed in vacuum while opposing each other, in particular, a field emission display using a carbon nanotube for an electron emission area of a cathode substrate.
2. Description of the Related Art
A field emission display including a cathode substrate with a use of carbon nanotubes (CNTs) is a thin display offering features including a high luminance, a wide viewing angle, a long operating life, a fast response, and a low power consumption. A research and development for the field emission display is now being in progress for its practical use.
Similar to a cathode ray tube (CRT), field emission displays (FEDs) are displays utilizing light emission (fluorescence) generated when accelerated electrons collide with a fluorescent member. A major difference between the FED and the CRT resides in that the FED has a part equivalent of an electron gun miniaturized to be arranged to each pixel while the CRT displays an image by subjecting an electron beam emitted from a single electron gun to scanning with a deflection yoke.
The FED has such a structure that a cathode substrate including arrays of field emission type emitters for emitting electrons and an anode substrate including a fluorescent plane are arranged in vacuum while opposing each other. The cathode substrate includes gate lines and cathode lines orthogonal to the gate lines, and the emitter array is formed at each intersecting point of the lines. The emitter is generally processed to have a shape with its tip end being shapely pointed.
Field concentration occurs at the shapely pointed tip end where a strong field can be accordingly applied. When a solid surface has a strong field applied thereto, electrons confined on the solid surface are likely to fly out in vacuum owing to the tunnel effect, and then a phenomenon called field emission occurs. In the FED, light emission is generated when electrons leaving the emitters due to the field emission collide with the fluorescent member on the anode substrate.
Known as such a device is an electron emission source having an improved electron emission effect by printing a paste containing CNTs on a cathode electrode, irradiating the printed surface with a laser beam, and selectively removing substances excluding the CNTs to expose the CNTs (refer to JP 2000-36243 A, for example).
The field emission is a phenomenon where electrons present in a substance are emitted in vacuum passing through a potential barrier owing to the tunnel effect with an applied high level field while employing the fact that the field is likely to concentrate at a position having a sharply pointed tip end as described above. In general, it is more difficult to control an electron emission amount in field emission element than a thermal electron emission element because “uniform fabrication of plural field emission elements is hard since each tip end is extremely pointed”, “a current density of field emission represented by the following Fowler-Nordheim equation has a higher sensitivity to the field than a current density of a thermal electron represented by the following Child-Langmuir equation”, and the like.
Formula 1
Fowler-Nordheim equation:
  j  =      a    ⁢                  ⁢          E      2        ⁢          exp      ⁡              (                  -                      b            E                          )            
Wherein j (A/cm2) is a current density, E (V/cm) is a field, and a and b are constants determined by a material and a geometric form of the emitter, respectively.
Child-Langmuir equation:
  j  =            8      9        ⁢          ɛ      0        ⁢                  e                  2          ⁢                                          ⁢          m                      ⁢                  V                  3          2                            d        2            
Wherein j (A/m2) is a current density, V (V) is an anode voltage, d (m) is a distance between the cathode and the anode, ε0 is the dielectric constant in vacuum, e is the elementary charge, and m is the electron mass.
For this reason, even when the emitter is expected to be fabricated in the same manner for each pixel in the FED, a slight difference in height and tip end curvature makes the electron amount generated from each emitter different upon application of the same voltage to the leading electrode. Thus, the luminosity among the pixels may fluctuate. In the case of the FED, it is unavoidable to some extent to use a driver circuit or the like to correct the luminosity fluctuation. However, if the fluctuation is at a low level, a circuit configuration etc. can be certainly simplified, which suppresses manufacturing costs.
On the other hand, as disclosed in JP 2000-36243 A, laser irradiation on the CNT printed layer removes components except the CNTs, making it possible to improve the electron emission efficiency. In JP 2000-36243 A, the electron gun for the CRT is supposed to function as an electron emission source. Electrons emitted from the single electron gun are subjected to scanning for irradiation to each pixel. Therefore, the luminosity fluctuation does not occur among the pixels. However, in the case of the FED, since each pixel is provided with an emitter as described above, it is necessary not only to improve the electron emission efficiency for each emitter, but also to make the electron amount generated from each emitter uniform and suppress the luminosity fluctuation among the pixel as much as possible. When laser irradiation for a wide area with a uniform energy density is carried out, a relatively uniform current can be obtained in view of comparison between wide areas. However, if areas each having a small area at a pixel level, e.g., about 0.2 mm×0.6 mm, are compared with each other, a large fluctuation occurs in their electron emission amounts based on a slight difference in laser irradiation results.